Ultrastructure of the developing muscle and enteric nervous system in the small intestine of human fetus.

The ultrastructural organization and some histochemical characteristics of the enteric nervous system (ENS) were investigated in 10- and 18-week-old human fetuses. In the 10-week-old human fetus immature myoblasts, and mostly neuroblasts were found in the ganglia. Simple, undifferentiated neuropil was observed among neuronal cells. The neuropil generally did not contain synapses; however axosomatic synapse was registered rarely on the surface of certain neurons. Neuromuscular junctions were common, both axons and neurons were in close contact with the sarcolemma. In the 18-week-old human fetus the fine-structural characteristics of the intestinal smooth muscle cells were the same as in the adult. Nerve profiles were frequently found among the muscle cells. NADH-diaphorase histochemistry revealed the presence of numerous ganglia but solitary neurons still occurred. Differentiated neurons and neuroblasts could be distinguished in the myenteric ganglia. Synapses were often detected in the neuropil. Thick nerve plexuses were frequently found in the proximity of smooth muscle cells, forming "distant" and "close" myoneural contacts. Well-defined fluorescent network and several fluorescent nerve cell bodies were demonstrated by glyoxylic acid. The above organization may provide a satisfactory basis for an integrated peristaltic movement in the gut of the 18-week-old human fetus.